Impregnated paper insulation for electric cables



A Feb. 13, A1940.

W. A. DEI. M AR 2,190,018

IMPREGNATED PAPER INSULATIONY FOR EI'JhC'l'vllICy CABLES Filed Aug. 22,193s ATTORNEY Patented Feb. 13, 1940 UNITED STATES IMPREGNATED PAPERINSULATION FOR ELECTRIC CABLES Application August 22, 1939, Serial No.291,313

4 Claims.

My invention relates to paper insulated cables, more particularly toimpregnated paper insulation; and has for its object to produce a cableoil which is non-sludging and will not crystallize out, be light coloredand possess valuable electrical characteristics not heretofore attainedwith rosin or other similar resins.

In preparing electrically insulated cables, it is customary to wrap theconducting metal wire with suitable paper or 'cellulosic material whichhas been impregnated with what is known as cable oil. This cable oil maybe a solution of rosin in a petroleum oil. 'Bhs oil must not sludge outnor separate on standing and it must be stable under conditions ofpreparation, storage and impregnation of paper. Wood rosin, althoughsoluble in paraiiin or naphthenic petroleum oils, often sludges outand/or crystallizes out on standing. This wood rosin is a commercialform of abietic acid and when used in cable oil itsfunctions are toassist the oil in "wetting the paper fibers, to increase its viscosityand to render the oil more stable under the influence of electricaldischarges. Rosin, however, has the effect of in' creasing the powerfactor of the oil at high temperatures and consequently ofthe cable, atthe higher operating temperatures. I have discovered that bysubstituting for rosin, a hydrogenated product of rosin, a low powerfactor of the oil is attained at all temperatures and that in addition,the stability of the oil under the influence of electrical discharges,isl greatly improved. This is particularly true with respect to powerfactor stability. For instance, one batch of bright stock paranin baseoil was mixed with 15% rosin and another with 15% hydrogenated rosin.Both werev subjected to electrical bombardment in a Nederbragt cell, asdescribed in Trans. A. I. E. E., 1937, vol. 56, pp. 1302-3, for 450minutes and the power factor measured before and after. 'I'he resultswere as follows:

Power factor at 100 C.

Oil with addition of- Before bom- After bombardment bardment 100 C'., 60C'. 100 C. 60 C. Wood rosin 6. 50 l. 50 10.00 0. 70 Hydrogenated rosin0. 55 0.05 l. 55 0. 13

Stability tests in the Nederbragt cell also give a measure of therelative merits of oils with respect to evolution of These tests showthat hydrogenated rosin 'materially retards the evolution of gas. Foringas under electric stress..

(Cl. 15b-2.6)

stance, the addition of 15% by weight of hydrogenated rosin to the oildescribed above reduces the gas evolution to 77% of the amount with pureoil. In this respect, however hydrogenated rosin is somewhat inferior towood rosin, the corresponding figure for which is 69%.

In Figure l, I show a Nederbragt cell test curve recording the gassingcharacteristics in relation to the per cent of hydrogenated rosin. Itwill be noticed that the stability against gassing in- 1Q creases withthe proportion of hydrogenated rosin. The curve shows the gassing in a450 minute run under glow discharge. Temperature of test=30 C., initialcarbon dioxide gas presr sure=l.0 millimeter of mercury, sixty cyclecur- 1 rent=1.0 milliampere.

The use of hydrogenated rosin, however is not merely a substitution ofhydrogenated rosin for ordinary rosin. The latter material increasesloil stability with increasing proportion of rosin up to at least 35%.Only the excessive viscosity of the compound limits the proportion toaround 15%. Hydrogenated rosin, however, if used on this basis wouldlead one into trouble as we have found that its usefulness as astabilizer is confined to a narrow range, as shown by Figure 2 whichshows a curve giving'the relation between the power factor stability ofoil compound in the Nederbragt cell test and the proportion ofhydrogenated rosin used in the oil. The power factor stability isindicated by the rise of power factor, measured at C., produced bysubjection to electrical discharge in the cell. The narrow dip in thecurve corresponds with the stable range of proportions of hydrogenatedrosin. I therefore make my cable with the proportions of hydrogenatedrosin corresponding with the dip in the curve.

Taking into account all the properties of the materials, I prefer to use10% hydrogenated rosin and of oil, but I obtain satisfactory resultswith proportions from 5 to 20% of hydrogenated rosin.

Cables made with 10% hydrogenated rosin compound have been subjected toaccelerated aging tests in comparison with cables made with 15% woodrosin compound and as expected from the Nederbragt cell tests, the cablepower factor readings show a greatly enhanced stability. While the riseof power factor in the rst ten load cycles does not differ very greatlyfrom that with wood rosin, subsequent load cycles show a much less riseof power factor.

Furthermore the cable goes through many more load cycles before failure.In a vtypical load cycle accelerated laging test, cable made with oiland wood rosin passed 2% power factor after 27 load cycles and failed atabout 30 load cycles. Cable with hydrogenated rosin in the oil, butotherwise identical has attained a power factor of only 1% after 80 loadcycles, Without any sign of injury. A remarkable fact is that while thepower factor of the cables with rosin kept on rising until failure,those with hydrogenated rosin have maintained a practically constantpower factor from the 30th to the 80th load cycle, indicating not onlygreater relative stability but something approaching absolute stability,something quite unprecedented with cables outside of the oil-lled andpressure types.

I claim: '1. An impregnated paper used for electric insulation forcables, comprising a. paper impregnated with oil containing in solutionfrom 5 to 20 per cent of hydrogenated wood rosin.

2. An impregnated paper used for electrical insulation for cablescomprising a paper impregtion from 5 to 20% of hydrogenated Wood rosin.,u

WILLIAM A. DEL MAR.

